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Table 1 Factors potentially influencing distribution of genetic variation across species ranges, their expected effect on genetic variability and the acting population processes.

From: Factors and processes shaping the population structure and distribution of genetic variation across the species range of the freshwater snail radix balthica(Pulmonata, Basommatophora)

Factor Expected effect on genetic variability in affected populations Population process
Geographic marginality negative Drift by increased population turn-over and low population density because of unfavourable environmental conditions
Gene-flow positive Gene-flow counteracts the effects of drift
Mixed mating system negative Selfing decreases effective population size and thus increases drift
Environmental marginality negative Drift by increased population turn-over and low population density because of unfavourable environmental conditions
Biotic interactions negative
or
positive
Drift by increased population turn-over due to interspecific competition
Introgression of alleles from related species by hybridisation
Dispersal barriers positive Accumulation of immigrating alleles
Range expansions negative Drift due to founder effects, bottlenecks or allele surfing
Population fragmentation negative Drift due to diminished effective population size
Secondary contact positive Mixing of alleles that evolved in isolation